Swelling of NaMg-montmorillonites and hydration of interlayer cations: a Monte Carlo study.

While the swelling behavior of laboratory-prepared homoionic montmorillonites has been studied extensively in numerous experimental and simulation works, far less attention has been given to much more abundant natural montmorillonites, containing a mix of monovalent and/or bivalent cations in interlayer spaces. We carried out a series of Monte Carlo simulations in order to investigate the reasons for the remarkable difference of experimental swelling patterns of a natural Na-rich/Mg-poor montmorillonite and a homoionic Na-montmorillonite. The simulations reproduced the swelling pattern of a natural montmorillonite, suggesting a mechanism of its hydration different from that of the homoionic montmorillonite. We also found that the differences in size and hydration energy of Mg2+ and Na+ ions have strong implications for the structure and the internal energy of interlayer water. This leads to a difference in the layer spacings of the simulated Mg- and Na-montmorillonites as large as approximately 2.1 A at lower water contents.